Ventricular fibrillation continues to be a major cause of sudden death. The objective of our research effort is to contribute to the development of improved standby automatic implanted defibrillation systems and subsystems which will be suitable for use in patients who are at high risk of suffering out-of-hospital episodes of ventricular fibrillation. Our immediate plans involve: (1) an intensive study of the effectiveness on a chronic basis of one-cycle, bidirectional, square-wave shocks when applied to epicardial electrodes implanted in 100 kg calves, (2) the development of suitable circuitry and hardware to implement such shocks with totally implanted apparatus, and (3) the evaluation of the operation of totally implanted systems in calves over an intermediate term (in the order of months).